Alkali metal organosilanolates and alkali metal organosiloxanolates can be used as initiators for base catalyzed ring opening polymerization of cyclic siloxanes to produce high molecular weight siloxane polymers. Primarily, potassium trimethylsilanolate is used to ring open cyclic trisiloxanes, cyclic tetrasiloxanes, and cyclic pentasiloxanes. Lithium organosilanolate salts have also been used to ring open cyclic trisiloxanes and cyclic tetrasiloxanes for the production of a narrow molecular weight distribution of siloxane polymers.
Another use for alkali metal organosilanolates and alkali metal organosiloxanolates is in the synthesis of siloxane bonds using a condensation type reaction. A generalized reaction scheme for such a process is shown below: EQU .tbd.Si--X+MO--Si.tbd..fwdarw..tbd.Si--O--Si.tbd.+MX
where M is a Group IA metal such as Na or K, and X is halogen. Thus, when M is Na and X is Cl, for example, the reaction is: EQU .tbd.Si--Cl+NaO--Si.tbd..fwdarw..tbd.Si--O--Si.tbd.+NaCl
Such silanolates and siloxanolates can be obtained by the reaction of diorganoalkoxysilanes with alkali metal hydroxides, or by the reaction of diorganosilanols with alkali metals or alkali metal hydroxides. These methods and their details can be found, for example, in British Patent 631,506 (Nov. 3, 1949), and in U.S. Pat. No. 3,641,090 (Feb. 8, 1972). Mono-alkali metal organosilanolates have also been produced by cleavage of the siloxane bond from the corresponding hexaorganodisiloxane compound, using an alkali metal hydroxide dissolved in an alcoholic solution. This procedure was reported by Hyde in U.S. Pat. No. 2,472,799 (Nov. 25, 1946); by Hyde, Johannson, Daudt, Fleming, Laudenslager and Roche in J. Amer. Chem. Soc. (75) 1953, 5615; and by Tatkock and Rochow in J. Amer. Chem. Soc. (72), 1950, 528. The synthesis of sodium triorganosilanolate using such a procedure is shown below: EQU 2NaOH+R.sub.3 Si--O--SiR.sub.3 .fwdarw.2NaOSiR.sub.3 +H.sub.2 O
U.S. Pat. No. 5,629,401 (May 13, 1997) and U.S. Pat. No. 5,637,668 (June 10, 1997) describe the synthesis of solvent soluble dipotassium organosiloxanolates. This was achieved by the equilibration reaction of cyclic siloxanes such as octamethylcyclotetrasiloxane or tetramethyltetraphenylcyclotetrasiloxane dissolved in cyclohexane solvent, with an aqueous solution of potassium hydroxide, where water was continuously removed by a distillation process. It is noted in the '401 and '668 patents that the synthesis of relatively short-chain sizes of soluble dipotassium organosiloxanolate polymers with a chain length of three siloxane units is also possible. Lithium hydroxide aqueous solutions were also used in the '401 and '668 patents for the preparation of dilithium organosiloxanolates by a similar procedure.
In contrast, and with respect to the present invention, a heterogeneous interfacial reaction is used to produce dialkali metal organosilanolate and dialkali metal organosiloxanolate salts. According to this method, a stoichiometric excess of an alkali metal hydroxide or an alkali metal oxide is used in a first phase, while a cyclic or linear siloxane polymer is used in a second phase. The first and second phases are immiscible with each other, so that the reaction occurs at the interface of the two phases, to produce a product which is then isolated using an extraction procedure.
The advantage of this present technique is that it can be tailored such that the product separates as a fine gel or as a suspension in a selected solvent, while excess unreacted alkali metal hydroxide and/or unreacted alkali metal oxide settles to the bottom of the reaction vessel. Isolation of the resulting silanolate or siloxanolate is a simple procedure in which the liquid gel is decanted, leaving residual alkali metal hydroxide and/or alkali metal oxide. Preferably, this is followed by solvent washing of unreacted alkali metal hydroxide and/or alkali metal oxide, and addition of the washings to the decanted gel. If desired, the gel can then be filtered to remove the suspended silanolate or siloxanolate, rewashed with a low boiling point solvent, re-filtered, and dried to obtain a final product.
However, because the dialkali metal organosilanolate or dialkali metal organosiloxanolate salt produced by this present procedure is a progressive degradation of a cyclic or linear polysiloxane reagent, it is necessary to ensure that the correct degree of degradation has occurred. In this regard, dialkali metal organosilanolates and dialkali metal organosiloxanolates formed by the process are alkaline dibasic salts which are readily soluble in water. Thus, they hydrolyze to produce two mole of hydroxy ions for each mole of salt. The OH.sup.- ions can be titrated with standardized hydrochloric acid to determine the alkali content. The titration yields information about the salt such as (i) alkali metal ion content, (ii) the K.sub.b or pK.sub.b of the salt, and (iii) the average number of silicon atoms in the dialkali metal organosilanolate or dialkali metal organosiloxanolate salt, i.e., the chain length of the salt.
In one preferred embodiment according to this invention, dipotassium dimethylsilanolate and dipotassium dimethylsiloxanolate are prepared by a heterogeneous reaction between solid, insoluble KOH, and a polysiloxane dissolved in toluene. The reaction is believed to occur by virtue of an initial cleavage of the siloxane bond by the hydroxy ion, to form a potassium silanolate group and a silanol group. The silanol group is then converted to a second potassium silanolate because of the presence of excess KOH.
This reaction scheme is shown below: ##STR1##
The reaction occurs progressively until the product separates out of solution as an insoluble dipotassium salt. Cyclic polysiloxanes used in the reaction degrade to single dipotassium dimethylsilanolates after about a 48 hour reaction period. Shorter reaction times lead to blends of dipotassium dimethylsilanolate and dipotassium dimethylsiloxanolate containing two silicon atoms. Linear siloxane polymers used in the reaction yield dipotassium dimethylsiloxanolates with 2-4 silicon atoms. Fourier transform infrared analysis of the reaction mix reveals that the rate of polysiloxane concentration reduction in the toluene solution is more rapid and more complete for the linear siloxane polymers than for the cyclic polysiloxanes. It is postulated that this occurs because linear siloxane polymers can more readily migrate onto the KOH surface than cyclic polysiloxanes.